1. Field of the Invention
The present invention relates to improved methods and means for effecting the production of sulfur-coated urea, and includes substantial improvements therein whereby substantially less sulfur needs to be utilized in the coating thereof than is taught to be utilized in the many processes disclosed in the prior art and yet there is achieved in the practice of the instant invention the same desirable low dissolution rate of the resulting urea as is achieved in the practice of even the best presently available art. It will be appreciated by those skilled in the art that the method of the present invention thusly results in significant cost benefits in the commercial production of sulfur-coated urea. It will also become readily apparent to those skilled in the art that still another benefit derived by the practice of the instant invention is that the resulting coated products contain substantially less sulfur for a given dissolution rate and thereby result in economically more attractive materials having higher nitrogen contents. Conversely, if desired, levels of sulfur coating now practiced by industry may be used in the practice of the instant invention to thereby yield sulfur-coated urea products demonstrating dissolution rates vastly lower than those that can be achieved by such presently best available art.
A principal consideration relating to the practice of the instant invention is the selection and use of methods and/or means wherein the modus operandi comprises the use of, as substrate material, granular or prilled urea that has been previously conditions with gelling clays generally known in the art as channelized 2:1 clay minerals and including attapulgite and sepiolite, as disclosed in the teaching of U.S. patent application Ser. No. 158,810, Elrod, filed Feb. 22, 1988.
2. Description of the Prior Art
At the present time, because of the state-of-the-art developed in view of and in response to certain considerations supra, there are available a number of methods and means which utilize in one way or another a plethora of approaches for applying coatings of sulfur to urea for the purpose of achieving controlled dissolution rates in the resulting product. It is generally agreed that at least one of the earliest and most economically attractive, as well as technologically sound, of these was that originally revealed by Blouin et al. in the parent application, now abandoned, of U.S. Pat. No. 3,295,950, Blouin et al., Jan. 3, 1967, assigned to the assignee of the present invention. Subsequently, certain improvements in his methods were developed and reported in U.S. Pat. No. 3,342,577, Blouin et al., Sep. 19, 1967, also assigned to the assignee of the present invention. More recently, U.S. Pat. No. 4,587,358, Blouin, May 6, 1986, assigned to the assignee of the present invention, has taught that the use of, in combination with urea, calcium lignosulfonate (or other metal or ammonium lignosulfonates), homogeneously incorporated therewith and therein, as a vastly improved conditioning material for imparting to the resulting urea both superior anticaking and nonfriable characteristics. Still more recently, in U.S. Pat. No. 4,676,821, Gullett et al., Jun. 30, 1987, also assigned to the assignee of the present invention, has taught the use of calcium lignosulfonate (or other metal or ammonium lignosulfonates) homogeneously incorporated with urea as a superior substrate for coating with sulfur, thereby resulting in the production of products having dissolution rates substantially lower than urea conditioned in the conventional manner with formaldehyde.
It is verily believed that to those skilled in this art, who study the disclosure of the present invention, it will be appreciated that the practice of same overcomes a great multiplicity of the limitations and disadvantages of the various methods and/or means of the type presently found in the prior art and that the present invention results in a new, novel, and unique combination of certain selected features of the advances in the art set forth by Blouin supra, albeit with wholly unexpected results. Accordingly, for purposes of teaching, disclosing, and claiming the instant invention, the teachings and disclosures of U.S. Pat. Nos. 3,295,950, 3,342,577, 4,587,358, and 4,676,821 supra are herewith and hereby incorporated herein by means of reference thereto. Also, for purposes of incorporation into the disclosure of the present invention teachings relating to a standard testing procedure to determine the 7-day dissolution rate, reference is hereby made to the general description of the dissolution rate procedure contained in U.S. Pat. No. 3,903,333, Shirley et al., Sep. 2, 1975, assigned to assignee of the present invention.